As thermal insulators, there have been known foam materials such as urethane foam and phenolic foam (foam-based thermal insulator). The foam materials exert the thermal insulating properties derived from their air bubbles generated by foaming. However, such urethane foam and phenolic foam typically have thermal conductivities higher than the thermal conductivity of the air. It is therefore of advantage to make the thermal conductivity of the thermal insulator be less than that of the air, for further improving the thermal insulating properties. As methods for achieving such thermal conductivities that are less than that of the air, there has been known a method of filling air-gaps of the foamed material (such as urethane foam and phenolic foam) with a gas having low thermal conductivities (e.g., chlorofluorocarbon), or the like. However, the method of filling air-gaps with the gas has a concern that the filled gas possibly leaks from the air-gaps over time, and which possibly causes increase in the thermal conductivities.
In recent years, there have been proposed vacuum-based methods for improving the thermal insulating properties. In the methods, for example, porous materials of calcium silicate and/or glass fibers are used and they are put in an airtight bag and maintained at vacuum state of about 10 Pa. However, the vacuum-based thermal insulating methods require the maintenance of the vacuum state, and thus have problems in temporal deterioration and production cost. Moreover, in the thermal insulator based on the vacuum, the shape of the thermal insulator would be restricted because it needs to maintain the vacuum state, and its application field is thus severely limited. Accordingly, the thermal insulator based on the vacuum has been limited in practical use.
In this regard, Patent Literature 1 discloses, as heat insulating structure having high intensity used for a vehicle body of aircraft or the like, body structure of aircraft having a foam layer serving as thermal insulator. However, in the structure disclosed in the patent literature, a heat insulating layer is made of resin foam material and has poor thermal insulating properties, and therefore increase in thickness of the heat insulating layer is required to achieve a heat insulating effect. Further, the heat insulating layer is formed on a framework by spraying, and therefore it is difficult to replace the heat insulating layer when its thermal insulating properties are decreased.